Ground engaging tools, such as bucket teeth of an excavator or a back hoe, ripper tips, and cutting edges of construction equipment that operate on gravel, sand and soil, require the tool to be able to withstand fracture, abrasion and wear. In order to have a good wear resistance, the tool must have high hardness and hardenability without being too brittle. In order to resist fracture, the tool must have high fracture toughness. In certain applications, such as mines or gravel pits, where the ground engaging tools are constantly impacted against rocks and gravel, the tools break more often because they do not possess the requisite high fracture toughness. In such applications, although good hardenability and high hardness are desirable, and more so for larger parts, they are however, not of as paramount an importance as fracture toughness. A number of attempts have been made heretofore, to provide a steel material that has extremely high fracture toughness.
In the past, other inventors have proposed a number of steel compositions having varying degrees of wear resistance and toughness. Most of these steel compositions include relatively large amounts, i.e. above 2%, of alloying elements such as chromium, nickel, molybdenum and silicon to improve the hardenability. For example, U.S. Pat. No. 3,973,951 issued Aug. 10, 1976 to K. Satsumabayashi et al., discloses a steel composition intended primarily for use as an excavating tool edge and having a chromium content of 3% to 6%. U.S. Pat. No. 4,170,497 issued Oct. 9, 1979 to G. Thomas et al., discloses a steel composition that preferably includes 3% to 4.5% chromium and is intended for use in mining buckets and other mineral processing operations. The steel composition embodying the present invention has excellent fracture toughness and good hardness, but it does not require alloying elements, such as chromium, to achieve high toughness. In fact, the steel composition embodying the present invention contains no more than 1.60% chromium, and preferably about 0.1% chromium. However, chromium may be added as a cost effective way to enhance the hardenability.
Other steels intended for use in applications requiring a combination of very high toughness and good hardenability require significant amounts of nickel. It is generally recognized that nickel imparts toughness and hardenability to steel because it does not form any carbides but remains in solution in the ferrite, thereby toughening the ferrite. U.S. Pat. No. 2,791,500 issued May 7, 1957 to F. Foley et al., U.S. Pat. No. 3,165,402 issued Jan. 12, 1965 to W. Finkl, and U.S. Pat. No. 3,379,582 issued Apr. 23, 1968 to H. Dickinson all disclose steel compositions having significantly large amounts of nickel. It has been discovered in the present invention that large amounts, i.e., above 150 ppm, of nitrogen can be used to achieve high fracture toughness in steels having very low amounts, i.e., below 0.1% of nickel. It has been further discovered that if nitrogen is present in such large amounts in balance with controlled amounts of certain other elements, particularly titanium, boron and niobium, the fracture toughness of the steel is remarkably enhanced. Thus, the steel composition embodying the present invention has excellent fracture toughness and good hardness, but it does not require nickel to achieve high toughness. In fact, the steel composition embodying the present invention contains no more than about 0.05% nickel. However, nickel may be added to enhance the hardenability.
It has been suggested by other inventors that the solubility of nitrogen in steel may be increased by increasing the amount of chromium in the steel. U.S. Pat. No. 5,232,660 issued Aug. 3, 1993 to Finkl et al. discloses a steel composition intended mainly for use in closed die drop forging die sets wherein the emphasis is to increase the hardness of steel to improve the wear resistance of the die and also to increase the hardenability of steel to improve the dimensional quality of the parts produced over the useful life of the die. Finkl uses nitrogen, in amounts ranging from 100 to 400 parts per million (ppm), in low alloy steels to primarily improve their hardness, hardenability and wear resistant properties. However, Finkl uses a substantially larger amount of nickel (0.6 to 1.0%), in an effort to increase the toughness of the steel. In the present invention, it has been observed that increasing the amount of nitrogen from a conventional value of about 80-90 ppm to a value in the range of about 150 to 400 ppm, and adding calculated amounts of titanium, boron and niobium, actually increases the fracture toughness of the steel but, unlike the suggestion made by Finkl, the hardenability of steel is not enhanced.
U.S. Pat. No. 5,131,965 issued Jul. 21, 1992 to McVicker and assigned to the same company as this instant invention, discloses a steel having good hardenability and toughness. However, McVicker uses high amounts of chromium (1.5 to 2.5%) and silicon (1.0 to 3.0%) to attain strength, hardenability and temper resistance properties without exploiting the effect of enhanced nitrogen in conjunction with titanium, boron and niobium to get high fracture toughness, as done in the present invention.
It has further been suggested by other inventors that the inclusion of aluminum and titanium in steel increases the wear resistance by increasing the hardenability due to the formation of nitrides and carbo-nitrides. Both the Finkl patent and U.S. Pat. No. 4,765,849 issued Aug. 23, 1988 to W. Roberts teach the inclusion of aluminum and titanium in the steel composition. Although the inclusion of titanium is similar to that proposed by the present invention, Roberts adds substantially higher amounts of aluminum (0.4% to 1.0%) than that specified in the present invention, to intentionally form aluminum nitride in the solidified steel product. Furthermore, Finkl appears to suggest that aluminum nitride is required to impart good toughness in steel. Contrary to the teaching in the Finkl patent, it is generally recognized that the presence of aluminum nitride is undesirable in steel requiring high toughness. Finkl also suggests that increasing aluminum and titanium will increase the solubility of nitrogen. The opposite of this is generally recognized i.e., although increasing the aluminum and titanium will increase the total nitrogen content in the steel, the nitrogen is tied up as precipitates and not as soluble nitrogen.
It is desirable to have an extremely high fracture toughness steel by exploiting the effect of nitrogen in conjunction with titanium, boron and niobium. It is further desirable that the steel composition be such that the high fracture toughness is always maintained independent of the hardenability and hardness, i.e., the chemistry that effects hardenability and hardness should be transparent to the base chemistry that yields high fracture toughness. The present invention is directed to overcome one or more of the problems as set forth above.